Prefab Spawning

Try this :

  var prefabToSpawn : GameObject;
  var spawnLocation : Vector3;
     
  function RunEvent () {
     Instantiate(prefabToSpawn, spawnLocation, Quaternion.identity);
  }

Then simply run the function at the desired time.

Like @Eric5h5 stated, Instantiate is what you will need. Heres an example using a trigger. When the Player enters the trigger we spawn 'somePrefab' at the players position and rotation.

 public var somePrefab:Transform;
 
 function OnTriggerEnter(other:Collider)
 {
     if(other.CompareTag("Player"))
     {
         Instantiate(somePrefab,other.transform.position,other.transform.rotation);
     }
 }